Method and system for calculating a height map of a surface of an object from an image stack in scanning optical 2.5D profiling of the surface by an optical system

ABSTRACT

Method and system for calculating a height map of a surface of an object from an image stack in scanning optical 2.5D profiling of the surface by an optical system, a focal plane is scanned at different height positions with respect to the object surface. An image is captured at each height position of the focal plane to form the image stack. The scanning of the focal plane comprises long range sensing and short range sensing a displacement of the focal plane for sensing low and high spatial frequency components. The height position of the focal plane is estimated by combining the low and high spatial frequency components. A height position of each image in the image stack is calculated, based on the estimated height position of each respective focal plane. The images of the image stack are interpolated to equidistant height positions for obtaining a corrected image stack.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 ofEuropean Application No. 17168243.8, filed on Apr. 26, 2017, thedisclosure of which is expressly incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The disclosure relates to a method and system in the field of scanningoptical 2.5D profiling of a surface of an object by an optical system.The disclosure more specifically relates to a method and system forcalculating a height map of a surface of an object from an image stackin such scanning optical profiling. The optical system is configured forscanning a focal plane at different height positions with respect to thesurface of the object, capturing an image at each height position toform the image stack, and calculating a height map of the object surfacefrom the image stack. Herein, scanning optical 2.5D profiling includestechniques like White Light Interferometry, WLI, Structured IlluminationMicroscopy, SIM, Point From Focus (synonym: Focus Variation) Microscopy,Confocal Microscopy, etc.

2. Description of Related Art

In optical scanning 2.5D profilometers, the optical sensor system isscanned with respect to a test sample. At each scanning position animage of the sample is recorded to generate a stack of images. Theseimages are processed to calculate a height map.

In most algorithms used to calculate a height map from an image stack, aconstant optical path length difference (step size) between all imagesis assumed, or at least the exact position at with each image was takenneeds to be known.

In reality, either because of servo errors or internal and externalvibration sources, there will be errors in the step sizes between theimages. These step size errors will in turn cause errors in theresulting height maps.

Herein, WLI is discussed as an example to explain these errors. Ininterferometry, a light beam is divided into two beams, a measurementbeam and a reference beam. When the two beams are recombined aninterference pattern (fringes) as function of the path length differencebetween the measurement beam and the reference beam can be recorded. Incase of perfectly coherent light with a narrow angular distribution theinterference pattern will stretch indefinitely. In such a case, solvingthe path length difference from the interference pattern is ambiguous.By reducing the spectral coherence (using a broad band light spectrum,also referred to as white light), the length of the interference pattern(coherence length) will be reduced, and the path length differencebetween the measurement beam and the reference beam can be determinedfrom the fringe pattern.

In the application of WLI for surface measurement of a sample, the focalplane is scanned vertically with respect to the sample surface. At eachfocus position an image is captured. The combined images form a 3D imagestack. Each lateral position of this image stack defines aninterferogram. From each interferogram, the local height position at thelateral position of the sample can be determined.

Most commercial WLI equipment is specified to measure height maps with arepeatability of only a few nanometer, nm.

The interference data analysis is done by using Fourier Transform, FT,methods. In the standard FT methods, a Fourier Transform or Fast FourierTransform of the recorded interference signal is calculated. As anexample, first a FT of the recorded interference signal may becalculated. Next, all frequency contributions except for the positivespatial frequencies corresponding to the illumination spectrum may beset to zero. By taking the inverse FT, the modulus of the obtainedsignal represents an envelope function of the original interferencepattern, whereas the argument represents the phase of the centralwavelength of the original interference pattern. The top of the envelopeis found by determining a centre of mass. This top can be used as afirst estimate of the local sample height. Next, the nearestphase-zero-crossing is found by linear interpolation of the unwrappedphase. The position corresponding with the phase-zero-crossing provideshighly accurate sample heights.

Others have used variations of FT methods, and also other methods. Thesemethods, however, all have nearly identical sensitivity fornon-equidistant scan steps.

The standard FT methods assume perfectly equidistant scan steps. If thescan steps are not perfectly equidistant there will be errors in theresulting height map. Three kinds of errors related to non-equidistantscan steps can be distinguished:

-   -   1. If the non-equidistant errors are small, this will result in        small offsets of both the envelope top and the        phase-zero-crossing. This causes waviness in the measured height        maps that corresponds to the height variation within the test        sample.    -   2. If the non-equidistance in the steps is large, the        phase-zero-crossing locating algorithm might snap to one of the        neighbouring phase-zero-crossings. This type of error appears as        discrete jumps in the height map with a magnitude of an integer        number times half of the central wavelength.    -   3. The local average scan step size might vary over a total scan        range. As the standard algorithm assumes a fixed scan step of,        for example, precisely 50, 60, or 70 nm, this effect will cause        scaling errors. As a result the height maps will either be        compressed or stretched. This effect will result in a systematic        error, for example when measuring step heights.

Thus, there remains a need for improvement of both the accuracy (stepheight measurements) and the repeatability of WLI measurements, inparticular when errors in the recorded position of images of the imagestack are caused by scan errors, vibrations or servo motor errors.

SUMMARY OF THE INVENTION

It would be desirable to provide an improved, or alternative method andapparatus to improve recorded positions of images of an object. It wouldalso be desirable to improve the accuracy of height map measurements inscanning optical 2.5D profiling. It would also be desirable to improvethe repeatability of height map measurements in scanning optical 2.5Dprofiling. It would also be desirable to improve the accuracy of heightmap measurements in case of scan errors, vibrations or servo motorerrors.

To better address one or more of these concerns, in a first aspect ofthe disclosure a method for calculating a height map of a surface of anobject from an image stack in scanning optical 2.5D profiling of thesurface by an optical system is provided. The method includes:

scanning a focal plane at different height positions with respect to theobject surface;

capturing an image at each height position of the focal plane to formthe image stack, wherein the scanning of the focal plane includes:

long range sensing a displacement of the focal plane for sensing lowspatial frequency components;

short range sensing a displacement of the focal plane for sensing highspatial frequency components; and

estimating the height position of the focal plane by combining the lowspatial frequency components and the high spatial frequency components;

calculating a height position of each image in the image stack, based onthe estimated height position of each respective focal plane;

interpolating the images of the image stack to equidistant heightpositions for obtaining a corrected image stack; and

calculating the height map of the surface of the object from thecorrected image stack.

By using two different types of displacement sensing, on the one hand along range sensing for sensing low spatial frequency components, and onthe other hand a short range sensing for sensing high spatial frequencycomponents of a displacement, the best estimate of a real distancebetween the object, in particular the surface of the object, and theoptical system corresponding to each focal plane can be calculated. Thelong range sensing is especially accurate in recording low spatialfrequency motion or displacement, whereas the short range sensing isespecially accurate in recording high spatial frequency motion ordisplacement. By combining the low spatial frequency components and thehigh spatial frequency components, an extremely accurate estimate of thedistance between the surface of the object and the focal plane of theoptical system (i.e., the height position of the focal plane withrespect to the surface of the object) can be obtained.

In this combining operation, filters may be used to select the mostappropriate spatial frequencies provided by the long range sensing andthe short range sensing. For example, the low spatial frequencycomponents measured by the long range sensing may be filtered to removehigh spatial frequency components. Alternatively or additionally, thehigh frequency components measured by the short range sensing may befiltered to remove low spatial frequency components. The filtering mayinclude using a moving average filter, or a filter in the frequencydomain using a Fourier transform, or any other appropriate digitalfunction.

The interpolating step includes interpolating the image stackintensities to an equidistant distance between all focal planes usingthe estimate of the height position corresponding to each focal plane.The interpolation may include a linear interpolation, a splineinterpolation, or a cubic interpolation.

In an embodiment of the method, the long range sensing includes optical,mechanical or capacitive encoding, in particular linear encoding.

The long range sensing may be performed using a long range typedisplacement sensor including an optical, mechanical, or capacitiveencoder, in particular a linear encoder.

In an embodiment of the method, the short range sensing includes sensingvibrations.

The short range sensing may be performed using a short range typedisplacement sensor including at least one vibration sensitive sensor,in particular an accelerometer or a Laser Doppler Vibrometer, LDV. In anembodiment, the LDV includes a system on a chip or a photonic integrateddevice.

In an embodiment of the method, the short range sensing is performedfrom the optical system, in particular from an objective thereof. Forthis purpose, at least one short range type displacement sensor ismounted to the optical system, in particular to an objective of theoptical system.

If it can be assumed that the object or object stage is less affected byvibrations than the optical system, then it may suffice to mount atleast one short range type displacement sensor to the optical system, inparticular the objective of the optical system. In some cases, this is afair assumption because of a combination of the stiffness and massinertia of a main frame and an object stage of an optical system and/orthe use of active vibration damping. Then, the short range sensingincludes sensing a displacement of the optical system with respect tothe object, or with respect to an object stage supporting the object.For this purpose, the short range type displacement sensor is adapted tomeasure short range type of displacement of the optical system withrespect to the object, or with respect to an object stage supporting theobject.

In an embodiment of the method, the short range sensing further isperformed from the object, or from an object stage supporting theobject, and the short range sensing is based on a difference of adisplacement sensed by the short range sensing performed from theoptical system and a displacement sensed by the short range sensing fromthe object, or from the object stage supporting the object. Accordingly,in addition to a short range type displacement sensor mounted to theoptical system, at least one further short range type displacementsensor is mounted to the object, or mounted to an object stagesupporting the object, wherein the short range sensing is based on adifference of the displacement measured by the short range typedisplacement sensor mounted to the optical system, and the displacementmeasured by the further short range type displacement sensor mounted tothe object, or mounted to the object stage supporting the object.

It may be beneficial to use additional short range type displacementsensors mounted to other parts of the system as well. For example,additional short range type displacement sensors may be added to a longrange type displacement sensor or part thereof, to an optical sensor, orto other places that are sensitive to vibrations. Accordingly, in someembodiments, more than one short range displacement sensor may bemounted to the object stage. Using predetermined vibration mode modelsof the object stage combined with a plurality of short rangedisplacement sensors, the precise vibrations of each location of theobject stage and thus of the object may be predicted.

A benefit of using accelerometers as short range type displacementsensors is that the properties, such as reflectivity, angle and/orroughness, of the surface of the object is not relevant for the qualityof the signal output by the accelerometers, in contrast to use ofoptical techniques wherein the surface properties of the object may bean issue adversely influencing signal quality.

Typically, the vertical resolution of an optical scanning profilometeris orders of magnitude better than the lateral resolution. Therefore,vibrations will mainly affect, referring to relative magnitude of errorswith respect to lateral and vertical resolution, the vertical directioncomponent of the resulting height maps. Therefore, in most cases animage stack correction in only Z direction suffices. However, albeit toa lesser extent, vibrations do affect the motion in the lateraldirection, in particular in an environment with strong vibrations suchas a shop floor with heavy machinery such as press drills, millingequipment, lathes, etc.

In most common implementations of 2.5D scanning optical profilers, onlyscanning in the vertical direction is performed. The correction in thelateral direction would not require a low spatial correction. Forlateral corrections only short range type displacement sensor signals,such as produced by an accelerometer or an LDV, would be sufficient fora correction. Furthermore, practice has shown that even in a worst case,a lateral shift of images of the image stack (thus, of the object withrespect to the optical axis) will be only one or at most two pixels in Xor Y direction.

To correct for lateral shift, in an embodiment, the method furtherincludes, before calculating a height map of the object surface from theimage stack, the steps of sensing a lateral displacement error of eachimage of the image stack and, if a lateral displacement error of any oneof the images is sensed, then laterally shifting or interpolating saidany one of the images to correct the lateral displacement error.

In a second aspect of the present disclosure, a system for calculating aheight map of a surface of an object from an image stack in scanningoptical 2.5D profiling of the surface is provided. The system includes:

an optical system configured for:

scanning a focal plane at different height positions with respect to theobject surface; and

capturing an image at each height position of the focal plane to formthe image stack;

a long range type displacement sensor for long range sensing adisplacement of the focal plane by sensing low spatial frequencycomponents;

a short range type displacement sensor for short range sensing adisplacement of the focal plane by sensing high spatial frequencycomponents, the short range type displacement sensor differing from thelong range type displacement sensor; and

a processing unit configured for:

estimating the height position of the focal plane by combining the lowspatial frequency components from the long range type displacementsensor and the high spatial frequency components from the short rangetype displacement sensor;

calculating a height position of each image in the image stack, based onthe estimated height position of each respective focal plane;

interpolating the images of the image stack to equidistant heightpositions for obtaining a corrected image stack; and

calculating the height map of the surface of the object from thecorrected image stack.

In an embodiment, the system includes at least one further short rangetype displacement sensor configured for sensing a lateral displacementerror of each image of the image stack, wherein the processing unitfurther is configured for, if a lateral displacement error of any one ofthe images is sensed, laterally shifting or interpolating said any oneof the images to correct for the lateral displacement error, beforeobtaining the corrected image stack. If an accelerometer based shortrange sensor is used, a 3D type of accelerometer integrated in onesystem, for example a MEMS, may be be used to measure high frequencydisplacements in X, Y and Z direction.

These and other aspects of the disclosure will be more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description and considered in connection with theaccompanying drawings in which like reference symbols designate likeparts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an embodiment of a system for generating animage stack in scanning 2.5D profiling of a surface of an object inaccordance with the present disclosure.

FIG. 1a depicts an interferogram, showing light intensity I against Zposition when scanning a location of an object.

FIG. 2 schematically depicts another embodiment of a system forgenerating an image stack in scanning 2.5D profiling of a surface of anobject in accordance with the present disclosure.

FIG. 3 depicts a block diagram of a controller of the system accordingto FIG. 1 or 2.

DETAILED DESCRIPTION OF EMBODIMENTS

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show structural details of the present invention in moredetail than is necessary for the fundamental understanding of thepresent invention, the description taken with the drawings makingapparent to those skilled in the art how the forms of the presentinvention may be embodied in practice.

Referring to the drawings wherein like characters represent likeelements, FIGS. 1 and 2 schematically depict, partly using a blockdiagram representation, an embodiment of a system in accordance with thepresent invention, including an interferometer apparatus depicted as aMirau interferometer. Alternatively, Michelson and/or Linnikinterferometer apparatus, or other optical profilers may be part of thesystem.

The system 100 for calculating a height map of a surface of an objectfrom an image stack in scanning 2.5D profiling of the surface of theobject includes a main frame 10 coupled to an XY stage 12 movableparallel to an X-Y plane oriented at right angles to a Z axis indicatedin FIGS. 1 and 2. The XY stage 12 is configured to support an object 14or test sample, and to move it parallel to the X-Y plane.

An optical system including an optical system frame 16 is coupled to themain frame 10 through a Z scanning motor 18 configured to move theoptical system frame 16 relative to the main frame 10 in oppositedirections parallel to the Z axis. A Z position of the optical systemframe 16 is sensed by a linear encoder pickup sensor 20 fixed to theoptical system frame 16, and sensing a linear encoder scale 22 fixed tothe main frame 10.

The optical system frame 16 includes a light source 24, or broadbandradiation source, being part of a broadband illuminator to provide abroadband illumination beam 26. The broadband illuminator furtherincludes a first mirror 28, and may include further optical equipmentsuch as lenses to provide the broadband illumination beam 26. Thebroadband illumination beam 26 may be parallel. The broadbandillumination beam 26 is reflected on a first beam splitter 30 and passesthrough an objective lens 32 to reach a second beam splitter 34 forsplitting the broadband illumination beam 26 in a reference beam 26 adirected to a reference mirror 36, and a measurement beam 26 b directedto the surface of the object 14.

The reference beam 26 a may be reflected on the reference mirror 36. Themeasurement beam 26 b may reflect from the surface of the object 14. Thebeam reflected from the reference mirror 36 may reflect again on thesecond beam splitter 34. The beam reflected from the surface of theobject 14 may pass through the second beam splitter 34. The referencebeam 26 a and the measurement beam 26 b may interfere to form aninterference beam 26 c and pass through the objective lens 32, the firstbeam splitter 30 and a lens 38 to an optical sensor 40, such as includedin a camera, including an array of pixel sensors. The intensity of theinterference beam 26 c may be measured by the optical sensor 40.

The reference mirror 36, the objective lens 32 and the second beamsplitter 34 together may form a Mirau objective and may be scanned withrespect to the object 14 in the Z direction along the optical axis ofthe objective lens 32 by means of the optical system frame 16 moved bythe Z scanning motor 18. Accordingly, the focal plane of the objectiveis scanned with respect to the object 14.

The signal of each of the pixel sensors of the optical sensor 40 may beread out to obtain an interferogram as depicted in box 50 in FIG. 1a ,which depicts a received intensity I as a function of the Z position Zof the corresponding surface location of the object 14.

The linear encoder pickup sensor 20 interacting with the linear encoderscale 22 forms a long range type displacement sensor 20 for long rangesensing of a displacement of the focal plane of the broadbandillumination beam 26 for sensing low spatial frequency components. Thelong range type displacement sensor 20 interacting with the linearencoder scale 22 is an optical encoder, in particular a linear opticalencoder. In other embodiments, a mechanical or capacitive encoder, inparticular a linear mechanical or capacitive encoder is used as a longrange type displacement sensor.

In the embodiment of FIGS. 1 and 2, the system 100 further includes afirst short range type displacement sensor 42 for short range sensing adisplacement of the focal plane for sensing high spatial frequencycomponents. The first short range type displacement sensor 42 differsfrom the long range type displacement sensor 20. The first short rangetype displacement sensor 42 includes at least one vibration sensitivesensor, in particular an accelerometer, as shown in FIG. 1, or a LaserDoppler Vibrometer, LDV, as shown in FIG. 2, or any other suitablesensor. An LDV may include a system on a chip or a photonic integrateddevice.

The first short range type displacement sensor 42 is mounted to theoptical system, in particular to the objective lens 32 of the opticalsystem, and is adapted to measure short range type of displacement ofthe optical system with respect to the object 14, or with respect to theXY stage 12 supporting the object 14.

According to the embodiment of FIG. 1, the system 100 may furtherinclude a second short range type displacement sensor 44 for short rangesensing a displacement of the focal plane for sensing high spatialfrequency components. The second short range type displacement sensor 44differs from the long range type displacement sensor 20 and from thefirst short range type displacement sensor 42. The second short rangetype displacement sensor 44 includes at least one vibration sensitivesensor, in particular an accelerometer or a Laser Doppler Vibrometer,LDV, or any other suitable sensor. An LDV may include a system on a chipor a photonic integrated device.

The second short range type displacement sensor 44 is mounted to the XYstage 12 supporting the object 14. In other embodiments, the secondshort range type displacement sensor 44 may be mounted to object 14.With the second short range type displacement sensor 44, the short rangesensing may be based on a difference of the displacement measured by thefirst short range type displacement sensor 42 mounted to the opticalsystem, and the displacement measured by the second short range typedisplacement sensor 44 mounted to the object, or mounted to the objectstage supporting the object.

As depicted in FIG. 3, the system 100 is provided with a computerprocessor/processing unit 60 for receiving several input signals,including a long range sensing signal 20 a from the long range typedisplacement sensor 20, a first short range sensing signal 42 a from thefirst short range type displacement sensor 42 and, if the second shortrange type displacement sensor 44 is present, a second short rangesensing signal 44 a from the second short range type displacement sensor44, and an image stack signal 70 a from an optical system 70. Theoptional presence of the second short range type displacement sensor 44is indicated by dashed lines.

The optical system 70 is configured for scanning a focal plane atdifferent height positions with respect to the surface of an object 14in scanning optical 2.5D profiling of the surface, and for capturing animage at each height position of the focal plane to form an image stack.The resulting image stack signal 70 a is output from the optical system70 to form an input signal for the processing unit 60.

The processing unit 60 is configured for, based on the input signals 20a, 42 a, possibly 44 a, and 70:

estimating the height position of the focal plane by combining the lowspatial frequency components from the long range type displacementsensor 20 and the high spatial frequency components from the (first)short range type displacement sensor 42, and possibly from the secondshort range type displacement sensor 44;

calculating a height position of each image in the image stack, based onthe estimated height position of each respective focal plane;

interpolating the images of the image stack to equidistant heightpositions for obtaining a corrected image stack; and

calculating the height map of the surface of the object 14 from thecorrected image stack.

The processing unit 60 may output the calculated high accuracy heightmap as a height map signal 80.

In the processing of the processing unit 60, the interpolating stepincludes providing equidistant images in height direction. Theinterpolating step includes interpolating the image stack intensities toan equidistant distance between all focal planes using an estimate ofthe height position corresponding to each focal plane. The interpolatingmay include performing a linear interpolation, a spline interpolation,or a cubic interpolation.

In most cases a correction of the image stack in the Z direction onlywith one-dimensional, 1D, short range type displacement sensors, such asaccelerometers or vibration sensors, will be sufficient. However, usingthree-dimensional, 3D, short range type displacement sensors arrangedfor sensing 3D displacements, or a combination of multiple 1D shortrange type displacement sensors sensing in different directions, allowsfor additional corrections in lateral directions to compensate forlateral movement in case of strong vibrations. The lateral directioncorrections applied to the image stack may be a simple translation by aninteger number of pixels in either X or Y direction. Alternatively, moreelaborate schemes including grid interpolation might be used. Theinterpolation may be either a simple linear interpolation or moreelaborate interpolation schemes. A correction in the Z direction wouldfollow. In such a case the image stack will be corrected in X, Y and Zdirection. The corrected image stack is then used to derive a heightmap.

In case lateral corrections are desired, at least one of the first shortrange type displacement sensor 42 and the second short range typedisplacement sensor 44, or at least one further short range typedisplacement sensor may be configured for sensing a lateral displacementerror of each image of the image stack. In such a case, the processingunit 60 may be further configured for, if a lateral displacement errorof any one of the images is sensed through a short range sensing signalfrom any one of the short range type displacement sensors, laterallyshifting or interpolating said any one of the images to correct thelateral displacement error, before obtaining the corrected image stack.

The processing of the processing unit 60 may include a filtering of ameasurement from the long range sensing to remove high spatial frequencycomponents. The processing of the processing unit 60 may further includea filtering of a measurement from the short range sensing to remove lowspatial frequency components. The filtering may include using a movingaverage filter, a filter in the frequency domain using a Fouriertransform, or any other suitable type of filtering.

As explained in detail above, according to the present invention acombination of two different types of motion/displacement sensors.

Low spatial frequencies are recorded using a long range typedisplacement sensor 20, for example a linear encoder. Additionalfiltering to remove high spatial frequency components may be applied.Suitable filters may be a moving average filter, or filters in thefrequency domain using Fourier methods, or any other appropriate digitalfunction.

High spatial frequencies are recorded using one or more short range typedisplacement sensors, such as accelerometers or Laser DopplerVibrometers, LDVs. Additional filtering to remove low spatial frequencycomponents may be applied. Suitable filters may be a moving averagefilter, or filters in the frequency domain using Fourier methods, or anyother appropriate digital function.

In case of using accelerometers, the recorded acceleration needs to beconverted to motion. This can be done by applying a double numericalintegration. Appropriate methods include the bar method, trapezoidintegration method, or any variation thereof.

Because of propagation of errors over time (as typically the scan speedis constant, distance is proportional to time), the displacementdetermined by accelerometers is only accurate within a short timespan/short distance. For longer range accuracy additional data using adifferent sensor would be needed.

As explained in detail above, in a method and system for calculating aheight map of a surface of an object from an image stack in scanningoptical 2.5D profiling of the surface by an optical system, a focalplane is scanned at different height positions with respect to theobject surface. An image is captured at each height position to form theimage stack. A height map of the object surface is calculated from theimage stack. The scanning of the focal plane includes long range sensingand short range sensing a displacement of the focal plane for sensinglow and high spatial frequency components. The low and high spatialfrequency components are combined for estimating a distance between thesurface of the object and the focal plane of the optical system. Aheight displacement error of each image in the image stack iscalculated, based on the estimated distance of each respective focalplane. At least one of the images of the image stack is corrected inheight direction, based on the associated height displacement error. Theheight map of the surface of the object is calculated from the correctedimage stack.

Herein, vertical scanning and a Cartesian coordinate system is assumed.However, the method and system as discloses herein is also applicablefor other configurations. For example, in engine borehole measurements,scanning is done axially with respect to the engine borehole. As such, acylindrical coordinate system is typically used. Other configurations,such as a rotated configuration, may be applicable as well.

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which can be embodied in variousforms. Therefore, specific structural and functional details ofembodiments disclosed herein are not to be interpreted as limiting, butmerely as a basis for the claims and as a representative basis forteaching one skilled in the art to variously employ the presentinvention in virtually any appropriately detailed structure. Further,the terms and phrases used herein are not intended to be limiting, butrather, to provide an understandable description of the invention.

The terms “a”/“an”, as used herein, are defined as one or more than one.The term plurality, as used herein, is defined as two or more than two.The term another, as used herein, is defined as at least a second ormore. The terms including and/or having, as used herein, are defined asincluding (i.e., open language, not excluding other elements or steps).Any reference signs in the claims should not be construed as limitingthe scope of the claims or the invention.

The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

The term coupled, as used herein, is defined as connected, although notnecessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of severalitems recited in the claims.

The terms software, program, software application, and the like as usedherein, are defined as a sequence of instructions designed for executionon a computer system. Software, a program, computer program, or softwareapplication may include a subroutine, a function, a procedure, an objectmethod, an object implementation, an executable application, an applet,a servlet, a source code, an object code, a shared library/dynamic loadlibrary and/or other sequence of instructions designed for execution ona computer system.

A computer program may be stored and/or distributed on a suitablemedium, such as an optical storage medium or a solid-state mediumsupplied together with or as part of other hardware, but also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems.

It is to be understood that the disclosed embodiments are merelyexemplary of the invention, which can be embodied in various forms.Therefore, specific structural and functional details disclosed hereinare not to be interpreted as limiting, but merely as a basis for theclaims and as a representative basis for teaching one skilled in the artto variously employ the present invention in virtually any appropriatelydetailed structure. Furthermore, the terms and phrases used herein arenot intended to be limiting, but rather, to provide an understandabledescription of the invention.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the present invention has been describedwith reference to exemplary embodiments, it is understood that the wordswhich have been used herein are words of description and illustration,rather than words of limitation. Changes may be made, within the purviewof the appended claims, as presently stated and as amended, withoutdeparting from the scope and spirit of the present invention in itsaspects. Although the present invention has been described herein withreference to particular structures, materials and embodiments, thepresent invention is not intended to be limited to the particularsdisclosed herein; rather, the present invention extends to allfunctionally equivalent structures, methods and uses, such as are withinthe scope of the appended claims.

The present invention is not limited to the above described embodiments,and various variations and modifications may be possible withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A method for calculating a height map of asurface of an object from an image stack in scanning optical 2.5Dprofiling of the surface by an optical system, the method comprising:moving to different height positions along a vertical axis, by actuatinga scanning motor, an optical system frame with respect to a main framealong the vertical axis; scanning a focal plane at the different heightpositions with respect to the surface of the object; at each heightposition of the focal plane: emitting a light towards the surface of theobject, splitting the emitted light into a reference beam and ameasurement beam, the reference beam being directed towards a referencemirror, and the measurement beam being directed towards the surface ofthe object, wherein the reference mirror reflects the reference beam tomeet with a reflected measurement beam, which reflects from the surfaceof the object, to form an interference beam, measuring an intensity ofthe interference beam using an optical sensor, and capturing, using theoptical sensor, an image to form the image stack, the image stack beingformed using images captured at the each height position of the focalplane, wherein the scanning of the focal plane comprises: long rangesensing a displacement of the focal plane for sensing low spatialfrequency components; short range sensing a displacement of the focalplane for sensing high spatial frequency components; and estimating theheight position of the focal plane by combining the low spatialfrequency components and the high spatial frequency components and basedon the intensity of the interference beam, calculating a height positionof each image in the image stack, based on the estimated height positionof each respective focal plane; interpolating the images of the imagestack to equidistant height positions for obtaining a corrected imagestack; and calculating the height map of the surface of the object fromthe corrected image stack.
 2. The method according to claim 1, whereinthe interpolating comprises performing one of a linear interpolation, aspline interpolation, or a cubic interpolation.
 3. The method accordingto claim 1, wherein the long range sensing comprises one of opticallinear encoding, mechanical linear encoding, or capacitive linearencoding.
 4. The method according to claim 1, wherein the short rangesensing comprises sensing vibrations.
 5. The method according to claim1, wherein the short range sensing is performed from an objective of theoptical system.
 6. The method according to claim 5, wherein the shortrange sensing comprises sensing a displacement of the optical systemwith respect to the object or with respect to an object stage supportingthe object.
 7. The method according to claim 5, wherein: the short rangesensing is performed from the object, or from an object stage supportingthe object, and the short range sensing is based on a difference of adisplacement sensed by the short range sensing performed from theoptical system and a displacement sensed by the short range sensing fromthe object or from the object stage supporting the object.
 8. The methodaccording to claim 1, further comprising filtering a measurement from atleast one of: the long range sensing to remove high spatial frequencycomponents; and the short range sensing to remove low spatial frequencycomponents.
 9. The method according to claim 8, wherein the filteringcomprises using a moving average filter, or a filler in the frequencydomain using a Fourier transform.
 10. The method according to claim 1,further comprising, before obtaining the corrected image slack: sensinga lateral displacement error of each image of the image stack; and uponsensing a lateral displacement error of any one of the images, laterallyshifting or interpolating said any one of the images to correct thelateral displacement error.
 11. A system for calculating a height map ofa surface of an object from an image stack in scanning optical 2.5Dprofiling of the surface, the system comprising: a scanning motor thatmoves, to different height positions along a vertical axis, an opticalsystem frame with respect to a main frame along the vertical axis; anoptical system that senses scanning a focal plane at the differentheight positions with respect to the surface of the object, at eachheight position of the focal plane: emits a light towards the surface ofthe object; splits the emitted light into a reference beam and ameasurement beam, the reference beam being directed towards a referencemirror, and the measurement beam being directed towards the surface ofthe object, wherein the reference mirror reflects the reference beam tomeet with a reflected measurement beam, which reflects from the surfaceof the object, to form an interference beam, measures an intensity ofthe interference beam using an optical sensor, and captures, using theoptical sensor, an image to form the image stack, the image stack beingformed using images captured at the each height position of the focalplane; a long range type displacement sensor that long range senses adisplacement of the focal plane by sensing low spatial frequencycomponents; a short range type displacement sensor that short rangesenses a displacement of the focal plane by sensing high spatialfrequency components, the short range type displacement sensor differingfrom the long range type displacement sensor; and a processor; and amemory including instructions that, when executed by the processor,cause the processor to perform operations including: estimating theheight position of the focal plane by combining the low spatialfrequency components from the long range type displacement sensor andthe high spatial frequency components from the short range typedisplacement sensor, and based on the intensity of the interferencebeam; calculating a height position of each image in the image stack,based on the estimated height position of each respective focal plane;interpolating the images of the image stack to equidistant heightpositions for obtaining a corrected image stack; and calculating theheight map of the surface of the object from the corrected image stack.12. The system according to claim 11, wherein the long range typedisplacement sensor comprises one of an optical linear encoder, amechanical linear encoder, or a capacitive linear encoder.
 13. Thesystem according to claim 11, wherein the short range type displacementsensor comprises at least one vibration sensitive sensor comprising oneof: an accelerometer; or a Laser Doppler Vibrometer comprising one of asystem on a chip or a photonic integrated device.
 14. The systemaccording to claim 11, wherein at least one short range typedisplacement sensor is mounted to an objective of the optical system.15. The system according to claim 14, wherein the short range typedisplacement sensor is configured to measure a short range type of adisplacement of the optical system with respect to the object or withrespect to an object stage supporting the object.
 16. The systemaccording to claim 14, further comprising at least one additional shortrange type displacement sensor mounted to the object or mounted to anobject stage supporting the object, wherein: the memory further causesthe processor to perform short range sensing based on a difference ofthe displacement measured by the short range type displacement sensormounted to the objective of the optical system, and the displacementmeasured by the at least one additional short range type displacementsensor.
 17. The system according to claim 11, further comprising atleast one additional short range type displacement sensor that senses alateral displacement error of each image of the image stack, wherein:the memory further causes the processor to perform, upon the sensing ofa lateral displacement error of any one of the images, laterallyshifting or interpolating said any one of the images to correct for thelateral displacement error, before obtaining the corrected image stack.18. The method according to claim 1, wherein the interference beam isformed at another beam splitter.
 19. The method according to claim 1,wherein lenses of a measurement device are fixedly positioned withrespect to the optical system frame.